I
have already stripped out the old plywood lining. It only came up as high as the top of
the steel bodywork, so it wasn't much use to me. Also, the floor was 18mm thick - much too
heavy duty for my needs.

What I'm planning to do is to re-line the walls and roof with 6mm
plywood and inject expanding polyurethene foam through strategically placed holes into the
voids behind. This is a much more suitable and, hopefully, cheaper method than using any
other insulation method. It will undoubtedly be the most effective